private void btnAdd_Click(object sender, EventArgs e)
{
String barang = tbProductID.Text + " " + nupQty.Value + " Piece(s) / " + tbProductName.Text + " - IDR " + tbPrice;


String id = tbProductID.Text;
int status = 0;
if (dgvProduct.Rows.Count > 0 && dgvProduct.SelectedRows.Count > 0)
{
if (nupQty.Value <= 0)
{
MessageBox.Show("You must buy at least 1 item!");
}
else if (nupQty.Value > int.Parse(tbStock.Text))
{
MessageBox.Show("There isn't enough stock for this product!");
}

else
{
for (int i = 0; i < lbCart.Items.Count; i++)
{
if (lbCart.Items[i].ToString().Substring(0, 5).Equals(id) == true)
{
MessageBox.Show("You cannot add the same item! To add more quantity, please delete the current item on the list!");
status = 1;
}
}
if (status == 0)
{
lbCart.Items.Add(barang);

}
}

}
else
{
MessageBox.Show("Please Choose a Product First!");
}
}